There have hitherto been disclosed in patents enormous methods with reference to processes for producing unsaturated carboxylic acids by vapor phase catalytic oxidation of unsaturated aldehydes, for example, a process for the production of methacrylic acid by vapor phase catalytic oxidation of methacrolein. These patents put emphasis on the method of producing acrylic acid from acrolein, and the method was impractical because of a low rate of selectivity due to severe side reaction and short life of the catalysts disclosed in the patents on using the catalysts as the ones for producing methacrylic acid.
On the other hand, many catalysts have been proposed for producing methacrylic acid from methacrolein. But these catalysts have problems that they are insufficient in reactivity, catalyst activity decreases greatly with the passage of time or high temperature is required for the reaction, so that a further improvement for the use as an industrial catalyst is required.